The present invention relates to a method of containment of a phase change material within a porous carbon material for the purpose of elevating the energy absorption capacity and performance of devices made with the material, such as brake disks and articles produced thereby. More particularly, the invention provides a method of incorporating and containing salts based on lithium within carbon/carbon composites, and particularly aircraft brake disks fabricated therefrom, in such a manner that the lithium salts undergo melting within an overload condition, absorbing energy from the overload. The molten lithium salts are constrained from escaping the porous carbon/carbon composite in the present system.
The function of aircraft brakes is to convert the kinetic energy of a rolling aircraft mass into thermal energy as the aircraft is brought to a rest. The ideal brake materials should exhibit a high coefficient of friction, be able to withstand the imposed mechanical stresses at elevated temperatures, and have a high specific heat. To minimize the weight liability associated with the braking function, the brakes should be as light as possible. Carbon-carbon composites offer a number of performance advantages over other brake materials in this application, and are becoming widely used for both military and civilian aircraft.
If aircraft brakes could be designed to handle only the loading associated with taxiing and normal landings, they could be made to be smaller and lighter than current designs, providing for lower operational costs for the aircraft over its service life. Unfortunately, the brakes must also be able to handle the loading associated with emergency braking. Such emergency braking produces a heat overload to the brake system. While the carbon/carbon composite material used for the frictional material in the brake can withstand high temperatures without structural degradation, friction coefficient or braking efficiency degrades at high temperatures and the remainder of the brake structure and wheel housing are susceptible to damage and even catastrophic failure from heat overload.
The most extreme of the braking overload conditions is represented by a rejected takeoff (RTO). An RTO is the perhaps once-in-an-aircraft-lifetime event in which the pilot of a fully-loaded and fully-fueled aircraft must abort its takeoff roll, and attempt to bring the aircraft to a full stop, using only the brakes. Such panic stops may require up to five times the thermal energy absorption of ordinary braking. Often the heat overload is so extreme that the entire brake and wheel assembly is damaged and must be replaced.
Currently, the approach for accommodating braking overload or an RTO is to make the brakes larger. However, the additional weight required to enable overload braking capacity represents a permanent loss of fuel and payload capacity for the aircraft. Since emergency or overload braking is an infrequent contingency condition, it would be desirable to develop lighter alternatives to accommodate this possibility.
One alternative that has been proposed to reduce the weight penalty associated with braking overload accommodation is the incorporation of phase change materials (PCMs) into the brake disk. The steel or carbon-carbon composite materials normally used for brakes absorb thermal energy as a product of their intrinsic specific heat, the mass, and the temperature increase experienced as a consequence of frictional contact. The advantage of incorporating PCMs into the brake system can be illustrated by considering the melting of ice, i.e., the change of phase from solid to liquid for H2O. The so-called heat of fusion of 333 Joules per gram must be added to ice at 0-C. in order to melt the ice into water. The temperature of the ice and water mixture will not increase until all the ice in the mixture is melted. By comparison, storing 333 Joules per gram of energy in liquid water raises its temperature 80° C., so the phase change effect is a dramatic way to store thermal energy without increasing the temperature of the material.
This phenomenon is generally true for phase changes in matter—PCMs absorb thermal energy, with no increase in temperature, as they change phase by means of their latent heats of fusion and vaporization. Thus, during the heating associated with an extraordinary braking event, a large amount of thermal energy could be absorbed by PCMs incorporated in the brake disks through either melting or vaporization while minimizing the temperature increase of the brake system.
The PCM brake concept entails minimizing the weight of the brake rotors and stators by increasing the heat capacity of the brake material. A cavity is machined or otherwise formed into the carbon/carbon material and PCM is inserted into a specialized insert within the cavity. During normal braking, the carbon-carbon material and solid phase PCMs provide sufficient thermal mass to absorb the braking energy without raising the temperature above the PCM liquidus. During extreme braking, the rotor/stator assembly gets much hotter. To protect the brake assembly, the selected PCM should melt, and absorb the heat of fusion during the phase change. After such extreme braking events, which can be expected several times during the life of the brakes, the PCM should ideally re-solidify within its cavity.
During an RTO, the temperature becomes so hot that the PCM would first melt and, most likely, then vaporize. Vaporization would result in pressure build-up within the volume containing the PCM, most likely resulting in diffusion of the vaporous PCM from its retaining volumes. Since an RTO currently causes heat build-up and damage to the entire wheel and brake assembly sufficient to warrant replacement of the assembly, loss of PCM in this circumstance would not entail any additional repair and maintenance.
Phase change materials (PCMs) have been utilized in a variety of thermal management solutions. Refrigerator coolants operate through the absorption and release of the heat of vaporization of the coolant, to pump heat from one part of the refrigeration system to another. In other applications of PCMs, the heat of fusion is utilized to absorb large quantities of heat with minimal increase in temperature to enhance performance of brake materials (see U.S. Pat. No. 5,139,118 to Schenk and U.S. Pat. No. 5,370,814 to Salyer).
However, attempts to fabricate a PCM brake assembly have not been completely successful. Moseley, et al., U.S. Pat. No. 5,613,578, suggests filling brake disk cavities with PCM, thus achieving higher heat capacity. However, the filled disk does not retain, restrict or contain PCM during phase changing, which can result in significant PCM loss. Clearly a high porosity medium, which can serve to retain, restrict or contain PCM during phase changing, is required.
Salyer, U.S. Pat. No. 5,370,814, teaches containment of PCM by mixing PCM with silica particles, which is not suitable for aircraft brake assembly because silica has low thermal conductivity, which would impair the heat flow from brake disk to PCM. Additionally, a silica/PCM mix does not take PCM volume expansion during phase changing into consideration, which can cause significant PCM loss when used in fixed cavity.
The prior art describes a potential improvement in the heat capacity of the brake, but does not specify how to package the phase change material so that it will be contained in a cavity within the brake disk. The salts of lithium (e.g., lithium fluoride, LiF; lithium metaborate, LiBO2; and lithium tetraborate, Li2B4O7) are candidate PCMs based on their melting temperatures, heats of fusion, and heats of vaporization. These salts also undergo a significant expansion upon melting from the solid state. Because of this, the PCM's are preferentially deposited within the inserts, leaving the necessary void to accomodate the material's expansion. Because carbon/carbon composites used in aircraft brakes are porous, the composites are permeable. Thus an expected outcome of a melting phase change in the salts of lithium is that air pressure on the inside of the cavity caused by the expansion of the PCM will force the liquid PCM through the walls of the carbon/carbon brake disk. The PCM would then be lost for future emergency brake operation, and repair and maintenance would be required. Also, liquid PCM could then come into contact with the wheel assembly, water vapor, de-icing fluids, and hydraulic fluids. Thus such loss of the PCM is destructive to the continued function of the brake.
Accordingly, the prior art clearly has several limitations, including the lack of a high thermal conductivity network interspersed with the PCM that distributes the energy created during braking uniformly and rapidly through the mass of the PCM. Without a high thermal conductivity network, the PCM will begin to vaporize at the heating surface before the bulk of PCM is melted. Another limitation of the prior art is the lack of containment of PCM as it is converted from an immobile solid phase to a mobile liquid phase. The prior art has no method of retaining the PCM as it melted, thus allowing the PCM to exit the brake through the porous carbon disk material.
Accordingly, there is still a need in the art for a method of containing a phase change material within a porous material for use in applications such as brake disks.